The present invention is directed to rare earth and/or transition metal doped Ca1+xSr1−xGayIn2−ySzSe3−zF2 (0≦x≦1, 0≦y≦2, 0≦z≦3) compounds that may be used for photon energy down conversion applications and the synthesis thereof.
Solid state lighting (SSL) technologies based on light emitting diodes (LEDs) are promising for a number of applications including general illumination, displays, medical systems, communication systems, etc. Significant growth in the SSL industry will be based on the availability of high efficiency, high power white LEDs. Currently available commercial white LEDs especially for warm white are not quite satisfactory for most general illumination applications. Their overall light output, luminous efficacy, color properties, and life must improve and the cost must be reduced before white LEDs can experience widespread usage in general lighting applications. Two popular methods for creating white light sources are (a) using phosphor based wavelength conversion structures and (b) using mixed color LEDs (red, blue and green referred to as RGB). Both these methods have their own advantages. The RGB based white LEDs offers the capability to tune colors in real time and better color properties in display applications. On the other hand, RGB white light LED systems require sophisticated active feedback control to keep the light at a stable color because the red, green and blue LEDs are created from different semiconductor materials. Currently the overall efficiency of RGB lighting system is low mainly due to low quantum efficiency of gallium indium nitride (Ga1−xInxN) direct emission green LEDs with peak emission wavelength near 555 nm (the peak of the human eye sensitivity). This is referred to as the “green gap” in the industry. To achieve high luminous efficacy for mixed color LEDs, the external quantum efficiency (EQE) of green LEDs needs to improve significantly. However, there are fundamental material challenges due to which high EQE for epitaxially grown Ga1−xInxN based direct emission green LEDs has not been achieved to-date. Phosphor-converted white light-emitting diodes (PC-LED) are rapidly progressing to meet the solid-state lighting goals of 200 lumens per watt (lm/W) by 2020 set by the United States Department of Energy (U.S. DOE). Presently available commercial white LEDs are delivering about 100 lumens per watt. However to reach 200 μm/W, significant improvements are needed at several stages, including internal quantum efficiency, extraction efficiency from the chip, and phosphor system efficiency, which includes phosphor conversion efficiency and extraction efficiency at the LED package level. Hybrid approaches for white light sources are also potential for general illumination purposes. In this approach, LEDs of individual wavelengths (red, blue, green, yellow, amber, etc.) with highest efficiencies are integrated into a system to provide color mixing. The individual wavelength LEDs may be either direct emission LEDs or PC-LEDs. In this regard, higher efficiency PC-LEDs for green emission wavelengths (in the green-gap) are better suited than the low efficiency direct emission green LEDs.
For display applications such as the Liquid Crystal Displays (LCD), LED based backlighting are anticipated to provide superior color gamut compared to the existing cold cathode fluorescent lamp (CCFL). Numerous benefits for LED backlighting lighting for LCD displays include: no mercury, much longer source life, greater than 30,000 hours, compared to CCFL, less prone to breaking. However, presently LED based displays are less energy efficient and higher in cost compared to CCFL based displays. Apart from the traditional general illumination and display technologies, there is a vast commercial market for LED based light sources with different emission wavelengths. Applications in biotechnology, indoor agriculture, photo-chemical reactions, adaptive illumination, photo-therapy, data communication, etc. are just a few examples.
For solid state light sources to be feasible for large scale deployment, there are few criteria that needs to be satisfied: higher wall plug efficiencies, low cost, availability of light sources with a variety of spectral content, ease of manufacturing and integration within systems, etc. Availability of light sources with any desirable peak emission wavelengths across the visible light spectrum will be necessary for a multitude of future applications. While direct emission LEDs based on semiconductor p-n junction diodes are available for discrete wavelengths, developing the technologies for high efficiency devices for a large number of emission wavelengths is not feasible. For direct emission LED development for any new emission wavelength, long term (5-10 years) and huge investments are necessary. In addition, integration and active control of large number of direct emission LEDs in a high efficacy light source is problematic and would be cost prohibitive as well as consume higher power during operation. PC-LEDs are attractive proposition since development of high efficiency phosphors of various emission wavelengths can be done simultaneously (short time period) with relatively low investments. Using the blue or ultraviolet (UV) direct emission Ga1−xInxN and Al1−xGaxN LEDs as excitation source for phosphors, PC-LEDs with large number of emission wavelengths may be developed. PC-LEDs also offer tremendous opportunities due to their simplicity and lower cost of fabrication, tunable and wide spectral characteristics, low power consumption and ease of operation, etc. Due to these reasons, intense research is being conducted world-wide in the area of down conversion phosphors that may be excited by blue LEDs.
High efficiency phosphors compounds have been studied extensively and sufficiently developed for UV excitation such as used in existing CFL (compact fluorescent lamp), CRT (cathode ray tube), CCFL (cold cathode fluorescent lamp), etc. However these phosphors have poor absorption and wavelength conversion efficiencies for excitation sources in the blue region of the visible spectrum (400-480 nm). Current research in new phosphor compounds is targeted towards the development of materials that possess high absorption coefficient for blue wavelengths and high quantum efficiencies for converting blue to longer wavelength photons. Rigorous search for high efficiency phosphor materials and unique composition of matter continues at the present time. Some of the high efficiency phosphor compounds found to-date are discussed below.
Phosphor-converted white LEDs are commonly achieved by using a yellow phosphor with a blue LED or by using red, green, blue (RGB) phosphors with a UV LED. One of the most popular yellow phosphors presently used in commercial white LEDs is Y3Al5O12:Ce3+ (YAG:Ce). Since the successful development of Ga1−xInxN blue LEDs, researchers have investigated four broad categories of high efficiency phosphors for white LED applications with various degrees of success. These high phosphors falls in the following categories: (i) metal oxides, (ii) metal sulfides, selenides and thiogallates, (iii) metal nitrides and (iv) metal oxo-nitrides. Some of these high efficiency blue wavelength excitable phosphors with emission peak tunable across the visible spectrum are already being used in white LED fabrication. The chemical compositions of these phosphors are listed below:
Yttrium aluminum garnet family: (YxGd1−x)3(AlyGa1−y)5O12: Ce3+, Pr3+ with 0<x<1.
Silicate garnet family: ML2QR4O12: Ce3+, Eu3+. Here M is elements from the group IIA (Mg, Ca, Sr, Ba). L is rare earth elements from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Q is elements from the group IVA (Si, Ge, Sn, Pb). R is elements from the group IIIA (B, Al, Ga, In, Tl).
Vanadate garnet family: Ca2NaMg2V3O12: Eu3+.
Mixed oxides family: (Y2−x−yEuxBiy)O3: Eu3+, Na2Gd2B2O7: Ce3+, Tb3+, YCa3M3B4O15: Eu3+ where M is elements from group IIIA (Al, Ga, In), LaCeSr2AlO5:Ce3+, Ba2Al2O4:Eu2+.
Alkaline earth metal silicates family: (Ba1−x−ySrxCay)SiO4:Eu2+ series such as Ca3MgSi2O8: Eu2+, Sr3MgSi2O8: Eu2+, Ba3MgSi2O8: Eu2+, and their mixtures; Ba2MgZnSi2O4:Eu2+, Sr3SiO5:Eu2+, Li2SrSiO4:Eu2+, and A2SiO4: Eu2+, D where A is elements from group II (Sr, Ba, Ca, Zn, Cd, Mg) and D is elements such as F, Cl, Br, I, N, S, P.
Alkaline earth metal sulfides and selenides, MS: Eu2+ and MSe: Eu2+. Here M is elements from group IIA (Mg, Ca, Sr, Ba) such as Ca1−xSrxS:Eu2+, Ca1−xSrxSySe1−y:Eu2+ with 0<x<1 and 0, y<1.
Alkaline earth metal thiogallates: metal sulfide thiogallates such as (SrMgCaBa)(GaAlIn)2S4:Eu2+ and metal sulfo-selenide thiogallates such as MA2(SxSey)4:B; MA4(SxSey)7:B; M2A4(SxSey)7:B; (M1)m(M2)nAp(SxSey)q; where M=Be, Mg, Ca, Sr, Ba, Zn; M1=Be, Mg, Ca, Sr, Ba, Zn; M2=Be, Mg, Ca, Sr, Ba, Zn; A=Al, Ga, In, Y, La, Gd; B=Eu, Ce, Cu, Ag, Al, Tb, Cl, Br, F, I, Mg, Pr, K, Na, Mn. The range of compositions covered for high efficiency sulfo-selenide thiogallate phosphors are as follows: m=0 to 1; n=0 to 1; m+n=1 (close to 1); p=close to 2 or close to 4; q=close to 4 or close to 7; when p=close to 2, q=close to 4; when p=close to 4, q=close to 7; x=0 to 1; y=0 to 1; x+y=0.75 to 1.25; x+y=0.5 to 1.5; B=0.0001 to 10 mole %.
Metal nitrides family: MxSiyNz:Eu2+, Ce3+ where M=Mg, Ca, Sr, Ba, Ln, Y, Yb, Al such as Sr2Si5N8:Eu2+, Ba2Si5N8:Eu2+, (Sr1−x−yBaxCay)2Si5N8:Eu2+, CaAlSiN3:Eu2+, CaxAlySizN3:Ce3+, CaSiN2:Ce3+.
Metal oxo-nitrides family: MSi2O2N2:Eu2+ where M=Ba, Sr, Ca, etc., (SrCa)p/2Alp+qSi12−p−qOqN16−q:Eu2+, (CaxMy)(Si,Al)12(O,N)16:Eu2+ where M=Eu, Tb, Yb, Er group element, LixMyLnzSi12−(m+n)Al(m+n)OnN16−n:Eu2+ where M=Ca, Mg, Y and Ln=Eu, Dy, Er, Tb, Yb, Ce, SrSiAl2O3N2:Eu2+.
According to the US Department of Energy (DOE) roadmap for phosphor development targets for 2015, quantum yield of 90% across the entire visible spectrum, color uniformity, color stability, thermal sensitivity and reduced optical scattering require the search for new phosphor materials and/or fine tuning the compositions of known phosphors. Therefore, it is the object of the present invention to synthesize selective crystalline phases of various alloy systems that have higher quantum conversion efficiencies and performance characteristics suitable for device fabrication and operation. It is a further object of the present invention to provide new alloy compositions that have been demonstrated to yield high wall plug efficiency and high efficacy light sources.